The present invention relates to an electronic timepiece provided with a temperature compensating capacitor.
Conventionally, X-cut quartz crystal resonators have had the frequency-temperature characteristics with a negative secondary temperature coefficient as shown in FIG. 1, and the timepieces incorporating the X-cut quartz crystal resonators have delays or time loses of 1.4 to 1.5 seconds/day maximum, in the temperature range of room temperature .+-.20.degree. C. even if the rate has been set at zero at room temperature. In order to improve the aberration of the rate, the frequency-temperature characteristics of the X-cut quartz crystal resonators have been compensated using a temperature compensating capacitor having a peak value at room temperature as shown in FIG. 2.
By the above compensation, the rate-temperature characteristics of the timepiece becomes plain as shown in FIG. 3 and the abberation of the rate is fixed around .+-.0.2 second/day maximum, at room temperature .+-.20.degree. C.
However, it is very difficult to fix the abberation of the rate around .+-.0.1 second/day at room temperature .+-.20.degree. C. by improving the rate-temperature characteristics of the capacitor because of the limitation of the capacitance-temperature characteristics of the temperature compensating capacitor.
Namely, the chief ingredient of the temperature compensating capacitor is generally BaTiO3, i.e., the Temperature of the Curie Point 120.degree. C. may be transfered to room temperature by applying some secondary ingredient. As a result, the capacitance changes on a large scale as shown in FIG. 2 taking advantage of the phase transition at the Temperature of the Curie Point (referred as TC hereafter).
The secondry ingredients are BaSnO3, CaSnO3, BaZrO3, SrTiO3 and the like. The ferroelectric ceramics comprises mainly of BaTiO3 have different capacitance-temperature characteristics at the lower temperature side and the higher temperature side centering around TC, though the capacitance-temperature characteristics are different more or less according to each secondary ingredient.
FIG. 4 shows the capacitance-temperature characteristics of BaTiO3-BaSnO3, BaTiO3-CaSnO3 and BaTiO-SrTiO3. Comparing the capacitance value of the capacitor at TC+20.degree. C. and at TC-20.degree. C, the capacitance at TC+20.degree. C. is larger than the capacitance at TC-20.degree. C. by 20 to 30% (TC=24.degree. C.) as shown in FIG. 4.
In the case that the temperature compensating capacitor having asymmetrical capacitance-temperature characteristics centering around TC is combined with the X-cut quartz crystal resonator having the frequency-temperature characteristics of the negative secondary temperature coefficient topped at the turn over point (referred to TP hereafter) as shown in FIG. 1, the temperature is not completely compensated since the frequency-temperature characteristics of the quartz crystal resonator is symmetrical centering around TP, whereby the rate at the lower temperature side advances and the rate at the higher temperature side delays or decreases as shown in FIG. 3.